Hundreds of families were fleeing from both sides of Pakistan’s border with Afghanistan Monday, officials said, as Islamabad continued a violent crackdown on extremists after multiple attacks last week raised fears of a militant resurgence.

Pakistan has accused Afghanistan of harbouring the militants who carried out last week’s attacks, which killed more than 100 people across the country.

The Pakistani military said it used heavy artillery to fire at militant hideouts in Afghanistan Monday, after carrying out airstrikes on both sides of the border over the weekend.

Journalists are not allowed into the area and the claims could not be independently verified.

But officials on the Afghan side said at least six people had been killed, and the Norwegian Refugee Council (NRC) said the firing had displaced up to 200 families.

“Military fire across the border into Nangarhar and Kunar Provinces has forced Afghan families to flee their homes… (such) attacks forcibly displace civilians, violating International Humanitarian Law, and must stop,” said NRC Country Director in Afghanistan, Kate O’Rourke.

A Pakistani security source told AFP Monday that up to 700 Pakistani families were being evacuated from along the border in Khyber tribal district “to protect them from any retaliatory attack”.

Kabul and Islamabad routinely accuse each other of providing safe haven to militants, and the Pakistani government openly admitted it shelters the Afghan Taliban leadership in statements made by the country’s top diplomat last year.

On Friday Pakistan gave Afghanistan a list of 76 “Afghan-based” militants and demanded that Kabul take action against them.

In a tit-for-tat move Monday, Afghanistan supplied Islamabad with a list of 85 militants it said were sheltering in Pakistan, calling for similar action.

Pakistan has already closed the two main gates along its porous border with Afghanistan, Torkham at the Khyber Pass and Chaman in Balochistan province. Officials said Monday they would remain closed “indefinitely”.

The spike in tensions was triggered by last week’s assaults, the deadliest of which was a suicide attack at a crowded Sufi shrine in Pakistan’s Sindh province on Thursday which killed 90 people, according to a new official toll, and which was claimed by the Islamic State (IS) group.

A series of other attacks were apparently coordinated by the Tehreek-e-Taliban Pakistan (TTP, or Pakistani Taliban), including a bombing in the eastern city of Lahore which killed 14 people and wounded dozens on February 13.

The emergence of IS and a TTP resurgence would be a major blow to Pakistan, which had enjoyed a dramatic improvement in security over the past two years after a military-led crackdown begun in 2014.

But critics have long argued that the government has not addressed the root causes of extremism, and analysts said there were “visible signs” militants were regrouping after last week’s attacks.